Mineral-detection apparatus

ABSTRACT

A marine mineral-detection apparatus is described including a 252Cf source of neutrons and a lithium-drifted germanium radiation detector shielded from the neutron source. The neutron source is mounted on an extensible member supported within a mass of neutron-shielding material. The source is extended outside the neutron shield to bombard surrounding mineral values with neutrons. Elements capturing neutrons give off prompt gamma radiation with discrete energies. The radiation detector resolves the radiation into distinct energy peaks for identifying the elements present in the mineral values.

[451 Jan. 25, W72

llnited States Patent Duffey et al.

[ MINERAL-DETECTION APPARATUS OTHER PUBLICATIONS [72] Inventors: DickDufiey, Adelphi; Peter F. W Fraenkel 3L; Properties of the Al theSpontaneous Fission of iggin pha Particles Emitted in Annapolis; FrankE. Senftle, Chevy Chase, y i Review er all Of Md. Oct. 5, 1964; pp. 438-441.

The

[73] Assignee: United States -of America as Primary Examiner-James W.Lawrence Assistant Examiner-Davis L. Willis represented by the UnitedStates Atomic Energy Commission May 26, 1970 Attorney-Roland A. Anderson[22] Filed:

ABSTRACT [21] Appl.No.: 40,644

ame e. h h m mT Tw 08 mmw mm am .nb .m mm d S 8 ed m ben 0 Hfl t .l[ gEr nd we d m d .mu .1 Swm w m m A marine mineral-detection a 2 diationdetector shielded neutron source is mounted on an extensible member sported within a mass of neutronsource is extended outside the neutronshiel Cf source of neutrons an [52] ILLS. ...250/83.3 R, 250/1068 [51]Int. Cl. G0lt 4/6 [58] Field ofSearch............250/83.l, 83.3 R, 83.6W, l06 S,

References Cited rounding mineral values with neutrons. Elementscapturing neutrons give off prompt gamma radiation with discrete ener-UNITED STATES PATENTS gies. The radiation detector resolves theradiation into distinct 6/1966 8/1969 3/ l 969 energy peaks foridentifying the elements present in the mineral values.

Carver et al.

Senftle et al.....

4 Claims, 2 Drawing Figures Bowman et al.

PATENTEB M25 197.2

INVENTORS F/PA/V/(E. .S'E/VFTLE BY DICK DUFFEY PETER E W/G'G'l/VS fl' 4.I ATTOR/T/[V Q,

, 1 MINERAL-DETECTION APPARATUS BACKGROUND OF INVENTION The presentinvention was made in the course of, or under, a contract with theUnited States Atomic Energy Commission.

FIELD OF THE INVENTION The present invention relates to mineralexploration devices employing neutron activation analysis. Thisinvention is particularly applicable to in situ exploration of the oceanfloor where it is expected that mineral values are disseminated asrelatively low-grade stratiform deposits. It may also have value in thegeneral field of geochemical mapping.

Neutron activation analysis can be used in a variety of ways to identifyunknown elements. One widely employed method is to convert the unknownelement into a short-lived radioisotope and monitor the radiationresulting from radioactive decay. Gamma radiation originating in thismanner will hereinafter be called activation or delayed gammas. Delayedgammas seldom have energy levels in excess of 1.5 mev. Thus the usefulactivated volume at the ocean floor is limited and widely disseminatedor randomlyscattered minerals may not be effectively detected fromdelayed gamma radiation. The presence of background radiation fromnatural emitters such as "K and the thorium-uranium series furthercomplicate delayed gamma radiation measurements. More significantly theabundant sodium and chlorine in ocean waters are easily activated byneutron radiation and the decay of their activation products tend tomask radiation from less concentrated minerals.

Another form of analysis with neutrons relies on prompt gamma detectionfor identification of neutron bombarded elements. When an elementcaptures neutrons a burst of capture gamma radiation is produced afterabout l to about 1f) seconds but prior to radiation from anyradioisotope decay. Generally, the slower neutrons are more likely to becaptured by a given element. Spectra signatures including a plurality ofcapture gamma peaks with distinct energies can be determined for variouselements and employed to identify these elements during mineralexploration. However, a gamma radiation detector with energydiscrimination to a few hundredth of an mev. must be employed to obtainuseful capture gamma signatures. Capture gammas are at energy levels ashigh as 9 mev. and therefore will reveal the minerals within a largersize sample than will delayed gammas which normally do not exceed about1.5 mev.

Use of fast neutrons for mineral detection will produce gamma radiationfrom inelastic scattering reactions as will as capture gammas. Both ofthese types of radiation occur and diminish prior to delayed gammas andaccordingly are generally termed prompt gammas. Inelastic scatteringgammas may have some value in mineral identification but can blur orconfuse capture gamma signatures. Consequently it is desirable tominimize the quantity of fast neutrons emitted from a neutron sourceemployed for capture gamma spectroscopy.

DESCRIPTION OF PRIOR ART Marine mineral exploration is presentlyaccomplished by physically removing samples from on or beneath the oceanfloor. One manner of obtaining samples is through use of hollow drillsections and recovery of the drill cores. Since many ocean minerals arebelieved to be in low concentration disseminated deposits, a largenumber of drill core samples at different locations may be required todetermine if a mineral field warrants mining and recovery costs.Consequently, such sampling methods are time consuming and expensive.

In general, neutron activation analysis to date has been based on thedetection of delayed gamma radiation in terrestrial mineral exploration.For example, see Californium- 25, Proceedings of a Symposium, pp. 32|-346, sponsored by the New York metropolitan Section of the AmericanNuclear Society, Oct. 22, 1968, J. J. Barker, Ed. USAEC CONF-68l032,.lan. l969. Application of delayed gamma radiation techniques to marinemineral exploration would be subject to the foregoing disadvantages anddifficulties.

SUMMARY OF INVENTION which can examine a large volume sample in searchof widelydisseminated mineral values.

It is a further object to provide a marine mineral detection apparatusfor locating deposits in the presence of background radiation andradiation from activated isotopes of sodium and chlorine.

In accordance with the present invention there is provided a Cf sourceof neutrons mounted on an extensible member within a body ofneutron-shielding material. The Cf can be extended outside the shieldingmaterial to irradiate surrounding mineral values with neutrons. Alithium-drifted germanium crystal for radiation detection is spaced andshielded from the Cf source. The crystal senses prompt gamma radiationproduced by the irradiated mineral values with sufficient discriminationof energy peaks to define energy spectra signatures for elementidentification. The crystal is contacted and cooled by multipleheat-conduction bars submerged at one end opposite the crystal in aliquid refrigerant.

DESCRIPTION OF THE DRAWINGS The present invention is illustrated in thedrawings wherein:

FIG. 1 is a partially cutaway perspective view of a mineral detectionapparatus;

FIG. 2 is a partially cutaway perspective view of a submersible vehiclefor lowering the mineral detection apparatus to the ocean floor. l

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG.1, a gamma ray detection unit 11 and a neutron source unit 13 are shownrigidly connected together by a suitable rod 15 and interlocked bands17. Units 11 and 13 are enclosed in generally spherically shaped housing12 and 14, respectively. Both housings l2 and 14 have projection ofreduced diameter 16 and 18, respectively, pointed downwardly forpenetrating surface sediment. A plurality of legs 19 are attached tobands E7 to support the units 11 and 13 on the surface where they are tobe used. Chains 21 are suitably attached to lower the units to the oceanfloor and to provide a retainer for power cables 23 entering each unit.

The neutron source unit 13 includes an encapsulated Californium-252neutron source 25. The Cf undergoes spontaneous fission to produce ahigher sustained neutron yield for a given source size than other knownneutron sources. Cf has an average neutron energy of 2.3 mev. such thatwith little moderation the quantity of inelastic scattering gammasproduced by this neutron source are minimized and capture gammasignatures can be clearly obtained. It is therefore particularly wellsuited for use in a mobile mineral detection unit for seeking widelydisseminated marine minerals. About 50 to 250 micrograms of Cf issufficient for the present application but larger or smaller amounts maybe used to vary the neutron flux.

The Cf is encapsulated in an impervious metal material to prevent lossof the valuable isotope. Suitable encapsulating materials includezirconium, niobium, and tantalum. These metals have insignificantcapture gamma spectra and are chemically and physically compatible withCf. Zirconium has only one relatively important capture gamma peak at6.29 mev. while niobium has two peaks which are not very prominent andtantalum has no capture gamma peaks of importance. Nevertheless, niobiumand tantalum absorb more neutrons and consequently their overall gammaray emission is higher than that of zirconium. Therefore in regard tosource background radiation, zirconium and zirconium alloys are thepreferred encapsulating materials of the three metals mentioned assuitable.

The encapsulated Cf neutron source 25 is shown supported by anextensible member 27 in its extended position for irradiatingsurrounding mineral values in geologic strata. One manner os extendingand retracting member 27 is to provide a reversible and rotaryservomotor 29 driving a helically coupled telescoping shaft 31. Member27 can be prevented from rotating with shaft 31 by a sliding pin andgroove connection (not shown) between member 27 and a nearby stationarywall The Cf neutron source 25, extensible member 27, and the servomotor29 are enclosed within an internal pressure vessel 33. Housing 14 maythereby be maintained at ambient pressure and thick exterior walls areunnecessary. Pressure vessel 33 and the walls of housing 14, especiallythe lower portions thereof, can be constructed of the recommendedencapsulating materials or of a plastic such as fiberglass to limitcapture gamma emissions from the construction materials. A hydrogenousneutron-shielding material 35, such as polyethylene, is provided in theremaining upper portion of housing 14 leaving the lower portion ofprojection 18 substantially free of neutron-shielding material. A thinlayer of hydrogenous or other neutron-moderating material (not shown)can be placed on the inside walls of projection 18 to reduce the energyof neutron radiation if the surrounding water will not be an adequatemoderator.

When storing or transferring the unit, the Cf is retracted into theupper central portion of vessel 33 and is substantially surrounded by auniform thickness of shielding material 35 to limit radiation emission.When the unit is in position with projection 18 penetrating surfacesediment, such as may be found on the ocean floor, the Cf is extendedinto the lower portion of projection 18 below shielding material 35 asshown. Surrounding geologic strata will thereby be irradiated withneutrons to produce capture gamma emissions from mineral valuescontained therein.

The gamma ray detection unit 11 includes a lithium-drifted germaniumcrystal 37 disposed in the hollow projection 16 of housing 12. Otherradiation detectors having gamma energy resolution substantially equalto that of the Ge(Li) crystal might also be used, e.g., having gammaenergy resolution to about l kev. for peaks in the mev. range. Crystal37 is encapsulated in a cadmium and lithium shield to prevent damageresulting from neutron radiation. A preamplifier 38 sufficientlyincreases the electrical signal from crystal 37 for transmission to arecording pulse height analyzer and spectrometer (not shown) in asupport vessel or vehicle.

A protective layer or coating of neutron-absorbing material 39 such asan aluminum-boron alloy is applied on the outside of housing 12. Thisprotective layer prevents gamma interference resulting from neutroncapture by the wall materials of housing 12. A layer of gamma shieldingmaterial 41, such as lead, is placed above the Ge(Li) crystal 37 toblock background and capture gamma radiation originating above the unit.

A reservoir 43 of liquid nitrogen or other suitable refrigerant isdisposed in the main body portion of housing 12. The refrigerantreservoir is spaced from the housing 12 wall to provide a peripheralchamber containing thermal insulation 45. Metal wires or fingers 49extend in a passageway 47 from the Ge(Li) crystal 37 through thepreamplifier 38 into refrigerant reservoir 43. The fingers transferexcess heat from the crystal 37 into the liquid refrigerant. Therefrigerant is maintained at a constant boiling temperature by apressure relief valve 51 in the wall of reservoir 43. The pressureinside housing 12 is maintained slightly above the outside pressure by asecond pressure relief valve 53. Thus as the unit is lowered into theocean the heat produced by the Ge(Li) crystal 37 provides pressurizationin the peripheral chamber containing insulation 45 to prevent collapseof housing 12. Supplemental heaters and remote pressure-monitoringdevices (not shown) may be provided to ensure sufficient pressure inhousing 12.

An attitude indicator 55 is placed within housing 12 to transmitelectrical signals indicating the orientation of the unit. The unit maythereby be positioned with the projections of reduced diameter 16 and 18always pointed downwardly into the ocean bottom.

In one manner of operating the apparatus of the present invention formarine mineral exploration, the detector and associated electronics arecalibrated on board the support vessel with capture gamma rays from purematerials. The unit is then further calibrated by lowering it into thewater to identify prompt gamma ray peaks emitted by sodium and chlorine.Signals from the attitude indicator are then used to position the unitupright on the ocean floor with the projections of reduced diameter 16and 18 penetrating the surface sediment. With the Cfsource 25 extended,the signals from the Ge( Li) crystal are counted and recorded. The seawater spectrum can then be subtracted from that obtained at the oceanfloor by raising the unit several feet about the surface and taking asecond reading. The net spectral signatures thus read from the detectorunit can be compared with known spectral signatures for suspectedelements and the minerals in a particular location at the ocean bottomidentified. Capture gamma signatures for several elements such as goldand manganese are given in the following publication by the inventors ofthe present mineral-detection apparatus: Mineral Exploration of theOcean Floor by In Situ Neutron Absorption Using A Californium-252Source," Marine Technology Society Journal, Vol. 3, No. 5, pp. 9-l6,Sept-Oct. 1969. As explained in this publication, a spectral signaturemay comprise several capture gamma peaks at distinct energy levels.Interference from another element will probably only obscure one of thepeaks making up the signature and the element is identifiablenotwithstanding the interference. For instance, manganese emits capturegamma radiation with peaks at 6.04, 6.24, 6.55 and 6.65 mev. Chlorinehas an interfering capture gamma peak at 6.64 mev,, but manganese canstill be identified from its remaining capture gamma peaks.

Another embodiment of the present invention is shown in FIG. 2. Asubmersible vehicle is shown having a pair of hollow projection 71 and73 extending from its bottom portion. A Cf source of neutrons 61 ismounted on an extensible member 63 for extending into and retractingfrom projection 73. Neutron-shielding material 65 surrounds theretracted position of the neutron source 61. A Ge(Li) radiation detector67 is disposed within projection 71 for sensing prompt gamma radiationproduced by neutron absorption into surrounding mineral values. Areservoir 69 of a liquid nitrogen provides refrigeration for cooling thedetector.

The submersible vehicle also includes a propulsion unit 75, acompartment for operators 77 and a second compartment 79 for theassociated electronics and data analyzer. The vehicle can proceedunderwater to locations where minerals are suspected to occur.Projections 71 and 73 can penetrate into the surface sediment while thevehicle rests on the ocean floor and readings are taken. The submersiblemay then be easily moved to examine numerous other locations.

A mineral-detection apparatus has been described for marine mineralexploration. The apparatus is kept mobile by use of a small size buthigh yield Cf neutron source. This. source irradiates a large sample ofgeologic strata to produce,

high-energy capture gamma radiation with minimum interfering gammas frominelastic scattering. A gamma radiation detector of high-energyresolution defines capture gamma spectra signatures with sufficientaccuracy to identify minerals in the presence of activated sodium andchlorine as well as other background radiation occurring beneath theocean surface.

We claim:

1. A marine mineral-detection apparatus which comprises,

in combination:

A. a first and a second housing member each having a generallyspherically shaped portion and a projection of reduced diameterextending therefrom, and structural means for supporting said housingmembers in fixed 6 spaced relationship with said projections of reducedtion of said second housing member for cooling said dediameter pointedtowards a common direction in axially tector, and parallel alignment; 3.neutron-absorbing material disposed between said Li- B. a neutron sourceunit disposed in said first housing drifted-Ge crystal and said neutronsource.

member i cl g 5 2. The apparatus of claim 1 wherein said neutron sourceI. a Cf neutron source,

2. hydrogenous shielding material disposed in the generally sphericallyshaped portion of said first housing member, and

3. extensible means for supporting said Cf neutron 10 source in aretracted position near the center of said hydrogenous shieldingmaterial, and in an extended position within said projection of reduceddiameter of said first housing member to bombard surrounding geologicstrata with neutrons;

C. a prompt gamma ray detector unit disposed in said second housingmember including 'g a gggg $2532?Li ia? zzg zg zfigfi g gz fig; 4. Themineral detection apparatus of claim 1 wherein said for producingelectrical pulses in response to prompt Cf neutron source isencapsulated in a material selected gamma radiation from mineral valueswithin Said from the group consisting of tantalum, l'llObllllTl,Zll'COlllUm geologic strata, and alloys thereof.

2. means disposed in the generally spherically shaped porunit includes asealed elongated vessel, containing said extensible means and said Cfneutron source, mounted within said first housing member and extendinginto said projection of reduced diameter.

3. The mineral-detection apparatus of claim 1 wherein said cooling meansincludes a. a closed reservoir of liquid refrigerant for cooling saidcrystal; and b. pressure relief valves within the walls of saidreservoir and said second housing member for discharging refrigerant gasand for maintaining said second housing at a pressure slightly greaterthan ambient pressure.

2. hydrogenous shielding material disposed in the generally sphericallyshaped portion of said first housing member, and
 2. means disposed inthe generally spherically shaped portion of said second housing memberfor cooling said detector, and
 2. The apparatus of claim 1 wherein saidneutron source unit includes a sealed elongated vessel, containing saidextensible means and said 252Cf neutron source, mounted within saidfirst housing member and extending into said projection of reduceddiameter.
 3. The mineral-detection apparatus of claim 1 wherein saidcooling means includes a. a closed reservoir of liquid refrigerant forcooling said crystal; and b. pressure relief valves within the walls ofsaid reservoir and said second housing member for dischargingrefrigerant gas and for maintaining said second housing at a pressureslightly greater than ambient pressure.
 3. neutron-absorbing materialdisposed between said Li-drifted Ge crystal and said neutron source. 3.extensible means for supporting said 252Cf neutron source in a retractedposition near the center of said hydrogenous shielding material, and inan extended position within said projection of reduced diameter of saidfirst housing member to bombard surrounding geologic strata withneutrons; C. a prompt gamma ray detector unit disposed in said secondhousing member including
 4. The mineral detection apparatus of claim 1wherein said 252Cf neutron source is encapsulated in a material seLectedfrom the group consisting of tantalum, niobium, zirconium and alloysthereof.